N the key to subsoil manuring success

Corinne Celestina says her PhD research has found yield boosts from subsoil manuring are probably because of better nutrition levels rather than from soil amelioration.

A PhD STUDENT has found yield increases from subsoil manuring are most likely a result of better nitrogen (N) levels rather than the soil amelioration of the deep ripping process.

Deep banding manure has seen some impressive yield benefits, but up until now there was little clarity as to why the yields were going up.

Corinne Celestina researched subsoil manuring for her PhD and found the major gains were from the added nutrition available to the plant.

She told more than 100 agronomists at the Incitec Pivot Fertilisers agronomy community forum last year that the positive results from subsoil manuring were largely due to better nitrogen nutrition.

“My take home message to agronomists and growers is to focus on crop nutrition first,” she said.

Subsoil manuring involves the incorporation of large amounts of nutrient-rich organic matter into the soil by deep ripping to improve its physical structure.

The process is mainly taking place in medium to high rainfall zones, where the soils often have constraints that block the flow of air and water into the root zone.

But rather than simply looking at subsoil constraints, Ms Celestina also checked whether nutrition levels were also a factor in keeping yields low.

In reviewing previous subsoil manuring trial results, Ms Celestina said the treatments which gave the best yield results were always the ones with the highest levels of plant nutrients, using amendments such as Dynamic Lifter or poultry litter.

But the design of the experiments did not allow her to determine whether it was the manure or the nutrients which improved crop performance.

This prompted Southern Farming Systems and Hart Field Site Group to establish new field experiments to assess crop yield responses to subsoil manuring.

The researchers added a fertiliser treatment which supplied nitrogen, phosphorus, potassium and sulphur to match the nutrients in the poultry litter.

High rates of nutrition were needed to match the nutrients supplied in the poultry litter – between 590-630 kilograms a hectare of nitrogen, 130-300 kg/ha of phosphorus, 270-400 kg/ha of potassium and 80-90 kg/ha of sulphur.

The treatments were also replicated at the surface to check whether there was a benefit in the deep placement.

The trials were conducted over eight sites from 2014 to 2016 on varying soils with subsoil constraints including alkalinity, sodicity, boron toxicity and low organic matter.

Ms Celestina told the Agronomy Community forum that these trials were “very different from the earlier subsoil manuring work”.

“We saw that subsoil manuring did not increase grain yields compared with other treatments,” she said.

The results also showed no response to tillage type, with the broadcast of poultry litter on the surface providing yields equal to the deep rip treatments.

The crops did not differentiate between the poultry litter and the matched nutrition from fertilisers, performing equally as well whichever was supplied.

“The yield results from the poultry litter were equal to using fertiliser with the same total nitrogen, phosphorus, potassium and sulphur rates,” she said.

“The carbon or microbial component of organic amendments did not show any advantage over chemical fertiliser.

“The yield increases we did see were most likely due to the alleviation of nitrogen deficiency.”

While Ms Celestina said structural changes in the soil likely did occur following subsoil manuring, they did not appear to be driving the yield increases.

“There was limited change in soil structure and microbes in the bulk of soil away from amendment lines, which suggests that plant roots are driving the changes,” she explained.